Method and memory device for dynamic cell plate sensing with AC equilibrate

ABSTRACT

A memory device that uses a dynamic cell plate sensing scheme. The memory device includes an array of word lines and complementary bit line/plate line pairs. A number of memory cells are located at the intersection of selected word lines and bit line/plate line pairs: A sense amplifier is coupled to the complementary bit line/plate line pairs. The memory device also includes an equilibrate circuit that ac equilibrates a complementary bit line/plate line pair at an equilibration voltage between high and low logic levels prior to reading data. The equilibration voltage and the high and low logic levels for the memory cell are chosen such that a fluctuation in the voltage on one of the plate lines does not corrupt data stored in unaccessed memory cells coupled to the same plate line.

This application is a continuation of U.S. Ser. No. 08/911,552, filedAug. 14, 1997 U.S. Pat. No. 5,862,089.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the field of electroniccircuits and, in particular, to a method and memory device for dynamiccell plate sensing with ac equilibrate.

BACKGROUND OF THE INVENTION

Electronic systems typically store data during operation in a memorydevice. In recent years, the dynamic random access memory (DRAM) hasbecome a popular data storage device for such systems. Basically, a DRAMis an integrated circuit that stores data in binary form (e.g., "1" or"0") in a large number of cells. Each memory cell includes a capacitorthat stores the data in the cell and a transistor that controls accessto the data. The capacitor includes two conductive plates. One plate ofeach capacitor is typically coupled to a common node with a plate ofeach of the other capacitors. This plate is referred to as the "cellplate." The charge stored across the capacitor is representative of adata bit and can be either a high voltage or a low voltage.

Typically, the cells of a DRAM are arranged in an array so thatindividual cells can be addressed and accessed. The array can be thoughtof as rows and columns of cells. Each row includes a word line thatinterconnects cells on the row with a common control signal. Similarly,each column includes a bit line that is coupled to at most one cell ineach row. Thus, the word and bit lines can be controlled so as toindividually access each cell of the array.

To read data out of a cell, the capacitor of a cell is accessed byselecting the word line associated with the cell. A complimentary bitline that is paired with the bit line for the selected cell isequilibrated with the voltage on the bit line for the selected cell.When the word line is activated for the selected cell, the capacitor ofthe selected cell discharges the stored voltage onto the bit line, thuschanging the voltage on the bit line. A sense amplifier detects andamplifies the difference in voltage on the pair of bit lines. Aninput/output device for the array, typically an n-channel transistor,passes the voltage on the bit line for the selected cell to aninput/output line for communication to, for example, a processor of acomputer or other electronic system associated with the DRAM. In a writeoperation, data is passed from the input/output lines to the bit linesby the input/output device of the array for storage on the capacitor inthe selected cell.

Recently, researchers have proposed a new architecture for sensing thevoltage stored in the cells of a memory device that is aimed atincreasing the voltage differential of the bit line pair. According tothis new architecture, the cell plate is divided into a number of linesthat are each paired with a bit line. The bit line/plate line pairs arecoupled to sense amplifiers which read and write data to and from thecells. One problem with this architecture is the difficulty inequilibrating the bit line/plate line pairs when reading data out of acell without destroying data stored in other cells on the same plateline since the voltage on the plate line is allowed to fluctuate.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora memory device that implements a dynamic cell plate sensing scheme withac equilibration wherein data is not corrupted during a read operation.

SUMMARY OF THE INVENTION

The above mentioned problems with memory devices and other problems areaddressed by the present invention and which will be understood byreading and studying the following specification. A method and memorydevice for a dynamic cell plate sensing scheme is described which usesac equilibration of bit line/plate line pairs wherein the high, low andequilibration voltage levels are chosen such that data stored in cellson a bit line/plate line pair are not corrupted when another cell on thebit line/plate line pair is accessed.

In particular, an illustrative embodiment of the present inventionincludes a memory device with an array of word lines and complementarybit line/plate line pairs. A number of memory cells are located at theintersection of selected word lines and bit line/plate line pairs. Asense amplifier is coupled to the complementary bit line/plate linepairs. The memory device further includes an equilibrate circuit that acequilibrates a complementary bit line/plate line pair at anequilibration voltage between high and low logic levels prior to readingdata. The equilibration voltage and the high and low logic levels forthe memory cell are chosen such that a fluctuation in the voltage on oneof the plate lines does not corrupt data stored in unaccessed memorycells that are coupled to the same plate line. In another embodiment,the voltage swing on the sense amplifier is greater than the voltageswing on the bit line/plate line pair between low and high logic levels.In another embodiment, p-channel isolation transistors are included tocouple the bit line and plate line to the sense amplifier so as to limitthe voltage swing on the bit line/plate line pairs. In anotherembodiment, a write-assist circuit is coupled between a bit line and aplate line and is coupled to the sense amplifier so as to decrease thetime for writing a low logic level back to one of the bit and platelines. In another embodiment, the sense amplifier includes an n-senseamplifier and a p-sense amplifier. A reference voltage source is coupledto the n-sense amplifier such that the n-sense amplifier drives one ofthe bit and plate lines to a reference voltage which reference voltagecorresponds to the low logic level for the memory cells. In anotherembodiment, the p-sense amplifier is coupled to a reference voltage thatis above the voltage of a supply voltage for the memory device so as toprovide sufficient voltage range to drive the sense amplifier.

In another embodiment, an apparatus that includes an electronic systemand a memory device is provided. The memory device uses dynamic cellplate sensing with ac equilibration of bit line/plate line pairs. Thehigh, low and equilibration voltage levels are chosen such that datastored in cells on a bit line/plate line pair are not corrupted whenanother cell on the bit line/plate line pair is accessed.

In another embodiment, a method for storing data in a selected memorycell of a memory device that uses dynamic cell plate sensing with acequilibration. Data is latched in a sense amplifier of the memory deviceis provided. The method converts logic levels of the data in the senseamplifier to different logic levels for the memory cells. The methodfurther stores the data in the selected cell. The logic levels for thememory cells are selected such that data in unaccessed cells that sharethe same plate line with the accessed cell are not corrupted. Furtherthe logic levels are selected such that the bit line/plate line pairsare equilibrated with ac equilibration without corrupting data.

In another embodiment, a method for reading and writing data in a memorydevice using a dynamic cell plate sensing scheme is provided. The methodprovides for equilibrating bit line/plate line pairs of the memorydevice with an ac equilibration. Further, the method provides swingingfirst and second nodes of a sense amplifier through a first voltagerange so as to latch data for reading from or writing to a cell of thememory device. Finally, the method provides for swinging bit line/plateline pairs through a second voltage range, different from the firstvoltage range such that data of cells that share a bit line/plate linepair are not corrupted when another cell is accessed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of a memory device that usesdynamic cell plate sensing according to the teachings;

FIG. 2 is a schematic diagram of a memory cell for use in the memorydevice of FIG. 1;

FIG. 3 is a schematic diagram of an embodiment of a sense amplifier fora memory device;

FIG. 4 is a schematic diagram of another embodiment of a sense amplifierfor a memory device;

FIG. 5 is a schematic diagram of another embodiment of a sense amplifierfor a memory device; and

FIGS. 6A through 6E are timing diagrams that illustrate voltage levelsfor signals used with the embodiment of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the illustrative embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific illustrativeembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense.

The illustrative embodiments described herein concern electricalcircuitry which uses voltage levels to represent binary logicstates--namely, a "high" logic level and a "low" logic level. Further,electronic signals used by the various embodiments of the presentinvention are generally considered active when they are high, however,an asterisk (*) following the signal name in this application indicatesthat the signal is negative or inverse logic. Negative or inverse logicis considered active when the signal is low.

FIG. 1 is a block diagram of an illustrative embodiment of the presentinvention. Electronic system 10 is coupled to memory device 12.Electronic system 10 comprises, for example, a microprocessor, memorycontroller, a chip set or other appropriate system that stores data in amemory device. Electronic system 10 is coupled to row decoder 14 ofmemory device 12 through address lines 16. Address lines 16 also coupleelectronic system 10 to column decoder 18. Control lines 20 coupleelectronic system 10 to control circuit 22. Finally, input/output lines24 couple electronic system 10 to input/output circuit 26.

Memory device 12 further includes sense amplifier 28 and array of memorycells 30. Array of memory cells 30 includes a number of word lines, WL-1through WL-M, a number of bit lines, BL-1 through BL-N, and a number ofplate lines, PL-1 through PL-N. Array of memory cells 30 is constructedso as to use a dynamic cell plate sensing scheme wherein each bit line,BL-i, is associated with a plate line, PL-i, to be used in reading andwriting data into a memory cell. To this end, bit lines BL-1 throughBL-N and plate lines PL-1 through PL-N are coupled in complementarypairs (referred to as "bit line/plate line pairs") to sense amplifier28. Further, word lines WL-1 through WL-M are coupled to row decoder 14.

Memory device 12 is controlled by control circuit 22. Control circuit 22is coupled to row decoder 14, sense amplifier 28, column decoder 18, andinput/output circuit 26.

Array of memory cells 30 includes a number of memory cells 32-11 . . .32-MN. Memory cell 32-11 is described herein by way of example. It isunderstood that the remaining memory cells are constructed in similarfashion. Memory cell 32-11 includes access transistor 34 and capacitor36. Access transistor 34 includes a gate that is coupled to word lineWL-1, a first source/drain region that is coupled to bit line BL-1 and asecond source/drain region that is coupled to a first plate of capacitor36. Plate line PL-1 forms the second plate of capacitor 36.

In operation, memory device 12 reads and writes data for electronicsystem 10. For example, to read the value from memory cell 32-11,electronic system 10 provides the address of memory cell 32-11 to rowdecoder 14 over address lines 16. Electronic system 10 also providescontrol signals to control circuit 22 over control lines 20. Controlcircuit 22 provides signals to sense amplifier 28 that causes anequilibrate circuit of sense amplifier 28 to equilibrate the voltages onbit line BL-1 and plate line PL-1. The equilibrate circuit of senseamplifier 28 forces bit line BL-1 and plate line PL-1 to a commonvoltage, e.g., approximately halfway between the high and low logicvalues for array of memory cells 30. It is noted that the range involtage between high and low logic levels for sense amplifier 28 differsfrom the range of memory cell 32-11. The advantages of this equilibrateprocedure are described below with respect to FIG. 2.

Row decoder 14 selectively drives word line WL-1 to a high logic levelto activate access transistor 34. When the voltage on word line WL-1 isa threshold voltage, V_(t), above the equilibrate voltage level, acharge stored on capacitor 36 is shared with bit line BL-1. For example,if a high logic level is stored on capacitor 36, the voltage on bit lineBL-1 increases. Additionally, by using plate lines in this manner,activation of access transistor 34 also changes the voltage on plateline PL-1 by an amount approximately equal in magnitude to the change onbit line BL-1, but opposite in direction. With the charge on the bitline/plate line pair, sense amplifier 28 next detects the logic state ofcell 32-11. Column decoder 18 receives the column address of theselected cell from electronic system 10. Column decoder 18 identifiesthe appropriate bit line/plate line pair for sense amplifier 28 to usein reading the value from memory cell 32-11. Sense amplifier 28 sensesand amplifies the difference in voltage in the bit line/plate line pairand thus produces high and low logic levels on complementary nodes ofsense amplifier 28 that correspond to the sensed bit line and plateline, respectively. These voltage levels are passed to electronic system10 through input/output circuit 26 over input/output lines 24.

In a write operation, electronic system 10 provides data to be writtento, for example, memory cell 32-11 over input/output lines 24 toinput/output circuit 26. Column decoder 18 receives the column addressfrom electronic system 10 over address lines 16 to select theappropriate bit line/plate line pair for thf selected memory cell. Senseamplifier 28, under the control of control circuit 22, forces the bitline/plate line pair for memory cell 32-11 to complementary high and lowlogic levels based on the data to be stored in memory cell 32-11. Rowdecoder 14 receives an address from electronic system 10 over addressline 16 that indicates the appropriate word line to activate for thisstorage operation. When word line WL-1 is activated, access transistor34 causes the data on bit line BL-1 and plate line PL-1 to be stored oncapacitor 36. In this process, the high and low logic levels for senseamplifier 28 are translated to appropriate voltage levels for memorycell 32-11.

A technical advantage of the present invention is the equilibration ofbit line/plate line pairs at a voltage level between high and low logiclevels that prevents the access transistor of a cell from inadvertentlyleaking voltage from the capacitor as the voltage on the plate linevaries when other cells that share the same plate line are accessed.

FIG. 2 is a schematic diagram of an embodiment of a memory cell,indicated generally at 40, according to the teachings of the presentinvention. In this example, the logic levels for cell 40 are set suchthat 2 volts is a high logic level and 1 volt is a low logic levelassuming a 2 volt power supply (V_(cc)) used for the memory device.Further, the bit and plate lines will be equilibrated to a voltage ofapproximately 1.5 volts which is halfway between the high and low logiclevels. This is in contrast to the conventional use of the power supply,V_(cc), for high logic level and ground for low logic level. It isunderstood that these voltage levels are given by way of example and notby way of limitation. Other appropriate logic levels can be substitutedwithout departing from the spirit and scope of the present invention.

Memory cell 40 includes access transistor 42 and capacitor 44. Accesstransistor 42 includes a gate that is coupled to word line WL, a firstsource/drain region that is coupled to bit line BL and a secondsource/drain region that is coupled to a plate of capacitor 44 at node46. Plate line PL forms the second plate of capacitor 44.

The operation of memory cell 40 is described to demonstrate theadvantage of using ac equilibration according to the teachings of thepresent invention. Initially, a voltage corresponding to a low logiclevel is stored in capacitor 44. To accomplish this, the voltage on bitline BL is brought to 1 volt and plate line PL is brought to 2 volts bya sense amplifier (not shown). Word line WL is activated such thattransistor 42 forces node 46 to a 1 volt level, a low logic level. Afterthe initial storage of the low logic level on capacitor 44, the bit lineBL and plate line PL are equilibrated to the equilibration voltage of1.5 volts each, e.g., during Row Address Strobe (RAS*) precharge time(tRP). Thus, node 46 reduces from 1 volt to 0.5 volts to maintain the 1volt difference on capacitor 44. Now, assuming that a high logic levelis read from another memory cell that is also coupled to bit line BL andplate line PL, bit line BL is raised to 2 volts and plate line PL isreduced to 1 volt. In memory cell 40, the voltage at node 46 is reducedto ground potential to maintain the 1 volt difference across capacitor44. At this voltage level, transistor 42 will not turn on (assuming thatword line WL is grounded when not active). If, however, node 46 isallowed to drop below ground, it could inadvertently affect the voltageon capacitor 44. Thus, by appropriate selection of the equilibration andhigh and low logic levels, the voltage at node 46 of memory cell 40 isassured of staying at a voltage level that will not inadvertently turnon transistor 42 and thus avoid corrupting the data stored on capacitor44 when another memory cell using the same bit line BL and plate line PLis accessed. Generally, the high logic level should be twice the lowlogic level if the equilibration voltage is halfway between high and lowlogic levels.

It is noted that the reduced voltage swing on the bit line BL and plateline PL could affect the speed and operation of a sense amplifiercoupled to the bit line BL and plate line PL pair. To improve the speedof operation of the sense amplifier, the sense amplifier can beconstructed so as to have a voltage swing between high and low logiclevels that differs from the voltage swing on the bit line/plate linepair. Several embodiments of the present invention that accomplish thisdifferent voltage swing in the sense amplifier and the memory cell aredescribed below.

FIG. 3 is a schematic diagram of a sense amplifier, indicated generallyat 100, that is coupled between memory bank A and memory bank B. Senseamplifier 100 is coupled to bit line BL-A and plate line PL-A pair atmemory bank A and to bit line BL-B and plate line PL-B pair at memorybank B. Sense amplifier 100 includes first and second equilibrationcircuits 102-A and 102-B, n-sense amplifier 104 and p-sense amplifier106. Sense amplifier 100 also includes first and second write-assistcircuits 108-A and 108-B. It is noted that write-assist circuits 108-Aand 108-B are not required but, as described below, are helpful inwriting back a voltage to the low bit or plate line. Finally, p-channelisolation transistors 110-A and 110-B couple bit lines BL-A and BL-B tonode 107, respectively, and p-channel isolation transistors 112-A and112-B couple plate lines PL-A and PL-B to node 105, respectively.

Equilibration circuit 102-A includes transistor 114 with a firstsource/drain region coupled to bit line BL-A, a second source/drainregion coupled to plate line PL-A and a gate coupled to receive anequilibration signal labeled EQ₋₋ A. Equilibration circuit 102-A furtherincludes first and second transistors 116 and 118. Transistor 116includes a first source/drain region that is coupled to bit line BL-A, agate that is coupled to receive the equilibration signal, EQ₋₋ A, and asecond source/drain region that is coupled to receive an equilibrationvoltage, V_(EQ). Second transistor 118 includes a first source/drainregion that is coupled to plate line PL-A, a gate that is coupled toreceive the equilibration signal, EQ₋₋ A and a second source/drainregion that is coupled to the voltage V_(EQ). When the EQ₋₋ A signal isat a high logic level, equilibration circuit 102-A effectively shortsbit line BL-A to plate line PL-A such that both lines are equilibratedto the voltage V_(EQ). Equilibration circuit 102-B is constructed in asimilar manner to equilibration circuit 102-A and operates when the EQ₋₋B signal is at a high logic level.

Sense amplifiers 104 and 106 comprise conventional n-sense and p-senseamplifiers constructed from n-channel transistors and p-channeltransistors in crosscoupled configurations, respectively.

Write-assist circuit 108-A includes first and second n-channeltransistors 120 and 122. Transistor 120 includes a first source/drainregion that is coupled to bit line BL-A, a gate that is coupled to node105 and a second source/drain region that is coupled to a node 124.Transistor 122 includes a first source/drain region that is coupled toplate line PL-A, a gate that is coupled to node 107 and a secondsource/drain region that is coupled to node 124 in common withtransistor 120. Further, write-assist circuit 108-A includes transistor126 that is coupled in a diode configuration with a gate coupled to afirst source/drain region and a second source/drain region coupled toground. The first source/drain region of transistor 126 is coupled to afirst source/drain region of transistor 128. Transistor 128 fuirtherincludes a second source/drain region that is coupled to node 124 incommon with transistors 120 and 122. Further, transistor 128 receives acontrol signal labeled WR₋₋ A at its gate. Write-assist circuit 108-B isconstructed in a similar manner to write-assist circuit 108-A andoperates when data is written back to memory bank B.

In operation, sense amplifier 100 is used to read and write data to andfrom memory bank A or memory bank B using a dynamic cell plate sensingscheme with ac equilibration. This embodiment uses a voltage swing onthe bit line and plate line at the memory bank that is different fromthe voltage swing of the sense amplifier. In other words, the valuesselected for the high and low logic levels on the bit line/plate linepair are different from the voltage swing of sense amplifiers 104 and106. This allows the bit line/plate line pairs to be equilibrated at avoltage that reduces the power consumption of the memory device and alsoallows the sense amplifier a sufficient voltage swing in order tooperate properly.

When data is read out of, for example, a cell of memory bank A, thegates of isolation transistors 110-B and 112-B are brought to a highvoltage level so as to isolate memory bank B from sense amplifiers 104and 106. Further, the ISO₋₋ A signal at the gates of isolationtransistor 110-A and 112-A is brought to a low logic level to couple bitline BL-A to node 107 and plate hine PL-A to node 105. Equilibrationcircuit 102-A is deactivated from bit line BL-A and plate line PL-A bylowering EQ₋₋ A to a low logic level. Subsequently, a word line inmemory bank A for the selected cell is brought to a high potential so asto share charge from a capacitor of the cell with bit line BL-A.Assuming a high logic level is stored, the voltage on bit line BL-Arises and the voltage on plate line PL-A lowers by a correspondingamount. Isolation transistors 110-A and 112-A transmit these voltages onbit line BL-A and plate line PL-A, respectively, to nodes 107 and 105 ofsense amplifiers 104 and 106. N-sense amplifier 104 forces the voltageat node 105 to ground potential upon activation of the signal labeledNLAT. Additionally, p-sense amplifier 106 drives the voltage at node 107to the power supply voltage in response to the signal labeled PSENSE*.Thus, sense amplifier 100 reads out a high logic value from the selectedcell of memory bank A.

When data is written back into, for example, memory bank A, a p-channelisolation device 110-A or 112-A translates the ground potential at node105 or 107 to a voltage of approximately 1 volt on the corresponding bitor plate line. Thus, use of p-channel isolation transistors 110-A and112-A allows sense amplifiers 104 and 106 to use a higher voltage swingthan bit line BL-A and plate line PL-A. This in turn allows the bit lineBL-A and plate line PL-A to be equilibrated to a voltage between the newhigh and low logic levels for the memory bank and reduce powerconsumption.

A further advantage of this embodiment is that it allows the memorydevice to operate at lower power supply voltages. This is due to thefact that the equilibration voltage is used to initially drive n-senseamplifier 104. The equilibration voltage needs to be high enough toovercome the threshold voltage of the transistors of an n-senseamplifier in order to turn it on. In a conventional DRAM, theequilibration voltage is a function of the high logic level and thus thehigh logic level (power supply voltage) can only be reduced so farbefore it produces an insufficient equilibration voltage for driving thesense amplifier. In this embodiment, the effect of lowering the highlogic valve (the power supply voltage) on the equilibration voltage istempered by the increase in the low logic level produced by thep-channel isolation transistors. Thus, for a given decrease in powersupply voltage, the embodiment will provide a higher equilibrationvoltage for n-sense amplifier 104 and the embodiment can therefore beused with lower power supply voltages than conventional DRAMs.

When opposite data is written back into memory bank A, sense amplifiers104 and 106 must switch states and drive the previously high node 105 or107 to a low logic level, e.g., ground, and the low node 105 or 107 to ahigh logic level, e.g., V_(cc). Continuing the example, assume a lowlogic value is to be stored in the cell of memory bank A. As the voltageon node 107 approaches ground, isolation transistor 110-A experiencesreduced drive current and thus slowly drives bit line BL-A to thedesired low logic level, e.g., 1 volt. In order to improve the speed atwhich bit line BL-A moves to an appropriate low voltage level, senseamplifier 100 includes write-assist circuit 108-A. The voltage at node105 moves toward V_(cc). This voltage is applied to the gate oftransistor 120 which turns on and passes the voltage generated bytransistors 126 and 128 to the bit line BL-A. Essentially transistors128 and 126 are fabricated to form, for example, a 1 volt power supply.Due to the use of n-channel transistors in this write-assist circuit108-A, the speed at which the bit line BL-A is reduced to the desired 1volt is increased. Thus, sense amplifier 100 advantageously usesp-channel transistors 110-A, 110-B, 112-A, and 112-B to provide adifferential between the swing in voltage on nodes 105 and 107 and theswing in voltage on plate line PL-A and bit line BL-A. Sense amplifier100 further advantageously increases the speed at which opposite data iswritten back to the memory bank by including write-assist circuit 108-Ato assist with writing back the low logic value to plate line PL-A orbit line BL-A. Write assist circuit 108-B provides the same advantagefor memory bank B.

FIG. 4 is a schematic diagram of another embodiment of a senseamplifier, indicated generally at 200, and constructed according to theteachings of the present invention. Sense amplifier 200 is coupledbetween memory banks A and B. Sense amplifier 200 includes first andsecond equilibration circuits 202-A and 202-B which are associated withmemory banks A and B, respectively. Sense amplifier 200 further includesisolation transistors 210-A and 210-B that couple bit lines BL-A andBL-B, respectively, to node 207. Further, sense amplifier 200 includesadditional isolation transistors 212-A and 212-B which couple platelines PL-A and PL-B, respectively, to node 205. Isolation transistors210-A, 210-B, 212-A, and 212-B allow memory bank A and memory bank B toshare common circuitry. Sense amplifier 200 further includes n-senseamplifier 204 and p-sense amplifier 206. N-sense amplifier 204 includesfirst and second n-channel transistors 222 and 224 coupled in across-coupled configuration. P-sense amplifier 206 includes first andsecond p-channel transistors 226 and 228 coupled in a cross-coupledconfiguration.

N-sense amplifier 204 is activated by a signal generated by n-channeltransistors 218 and 220. Transistor 218 includes a first source/drainregion that is coupled to node 230 of n-sense amplifier 204. Transistor218 further includes a gate that is coupled to receive a control signallabeled NSENSE. Transistor 220 includes a first source/drain region thatis coupled to ground. Additionally, a second source/drain region oftransistor 220 is coupled to a gate of transistor 220 and a secondsource/drain region of transistor 218. Thus, transistor 220 is coupledin a diode configuration and provides a fixed voltage reference of, forexample, 1 volt.

P-sense amplifier 206 is activated by a signal generated by transistors214 and 216. Transistor 214 includes a first source/drain region that iscoupled to node 232 of p-sense amplifier 206. Transistor 214 furtherincludes a gate that is coupled to receive a control signal labeledPSENSE*. Transistor 216 includes a first source/drain region that iscoupled to a pumped power supply labeled V_(ccx) that is above the powersupply voltage, V_(cc), by at least a threshold voltage of isolationtransistor 210-A. Additionally, a second source/drain region oftransistor 216 is coupled to a first source/drain region of transistor214. Transistor 216 also receives a signal, WT, which provides thevoltage V_(ccx) to the transistor 214.

By using n-channel transistors as the isolation transistors in thisembodiment, p-sense amplifier 206 can drive nodes 205 or 207 to acharged voltage value, e.g., V_(ccx), and the isolation transistors willonly pass a value of V_(cc) to the appropriate bit line or plate line asa high logic value. Further, transistors 218 and 220 combine withn-sense amplifier 204 so as to pull down either node 205 or 207 to avoltage of approximately 1 volt. The isolation transistors pass thisvoltage level to the bit line or plate line as the low logic value forthe memory cells. Thus, the sense amplifier is able to have sufficientvoltage swing (e.g., from 1 volt to V_(ccx)) in order to operateproperly and the bit line/plate line pairs are able to operate at areduced voltage swing (e.g., from 1 volt to V_(cc)) so as to allowdynamic cell plate sensing to be used with ac equilibration.

In operation, sense amplifier 200 is used to read and write data to andfrom memory bank A or memory bank B using a dynamic cell plate sensingscheme. When data is read out of, for example, a cell of memory bank A,the ISO₋₋ B signal at the gates of isolation transistors 210-B and 212-Bis brought to a low voltage level so as to isolate memory bank B fromsense amplifiers 204 and 206. Further, the ISO₋₋ A signal at the gatesof isolation transistors 210-A and 212-A is brought to a high logiclevel to couple bit line BL-A to node 207 and plate line PL-A to node205. Equilibration circuit 202-A is deactivated from bit line BL-A andplate line PL-A by lowering EQ₋₋ A to a low logic level. Subsequently, aword line in memory bank A for the selected cell is brought to a highpotential so as to share charge from a capacitor of the cell with bitline BL-A. Assuming a high level is stored in the cell, the voltage onbit line BL-A rises and the voltage on plate line PL-A lowers by acorresponding amount. Isolation transistors 210-A and 212-A transmitthese voltages on bit line BL-A and plate line PL-A, respectively, tonodes 207 and 205 of sense amplifiers 204 and 206.

N-sense amplifier 204 forces the voltage at node 205 to a voltage ofapproximately 1 volt upon activation of the signal labeled NSENSE. Asthe voltage between node 230 and node 207 approaches a thresholdvoltage, V_(t), transistor 224 begins to conduct. Conduction results inthe discharge of the low voltage node toward the voltage established bytransistors 218 and 220. In our example, this voltage is approximately 1volt. Ultimately, node 230 will reach the 1 volt level, bringing node205 with it. Note that transistor 222 will not conduct since its gatevoltage derives from node 205, which is discharging toward 1 volt.

Shortly after n-sense amplifier 204 is activated, signals PSENSE* and WTare driven toward ground. This activates p-sense amplifier 206 thatoperates in a complementary fashion to n-sense amplifier 204. With node205 approaching ground, a strong signal exists to drive transistor 226into conduction. This conduction charges node 207 to a high voltage,V_(ccx).

Data is written back to memory bank A by latching the data with senseamplifiers 204 and 206 from an external source onto nodes 205 and 207.In this process, transistor 216 is turned off momentarily to allow senseamplifiers 204 and 206 to flip logic states. Further, isolationtransistors 210-A and 212-A transmit this data to bit line BL-A andplate line PL-A. As mentioned above, the node 205 or 207 thatcorresponds to the high logic level will be reduced by the isolationtransistor down to approximately the power supply voltage V_(cc).

FIG. 5 is a schematic diagram of another illustrative embodiment of thepresent invention. Sense amplifier 300 is coupled between memory banks Aand B. Sense amplifier 300 includes equilibration circuits 302-A and302-B. Sense amplifier 300 includes first and second isolationtransistors 310-A and 310-B that couple bit lines BL-A and BL-B,respectively, to node 307. Additionally, sense amplifier 300 includesn-channel isolation transistors 312-A and 312-B that couple plate linePL-A and PL-B, respectively, to node 305. Further, memory banks A and Bshare n-sense amplifier 304 and p-sense amplifier 306.

P-sense amplifier 306 includes first and second p-channel transistors326 and 328 coupled in a conventional cross-coupled configuration. Anadditional p-channel transistor 314 includes a first source/drain regionthat is coupled to node 332. Additionally, a second source/drain regionof transistor 314 is coupled to a power supply, V_(cc). A gate oftransistor 314 is coupled to a control signal PSENSE* that activatesp-sense amplifier 306.

N-sense amplifier 304 includes first and second transistors 322 and 324coupled in a cross-coupled configuration. Additionally, n-senseamplifier 304 includes transistors 318, 340 and 342. Transistor 342 iscoupled in a diode configuration so as to provide, for example,approximately a 0.7 volt reference voltage. Transistor 340 includes afirst source/drain region that is coupled to a first source/drain regionof transistor 342. Additionally, a gate of transistor 340 is coupled toreceive a control signal labeled NLAT2. Additionally, a secondsource/drain region of transistor 340 is coupled to a first source/drainregion of transistor 318. A second source/drain region of transistor 318is coupled to ground and a gate of transistor 318 is coupled to receivea control signal labeled NLAT1. The first source/drain region oftransistor 318 is also coupled to node 330 in order to drive n-senseamplifier 304.

In operation, n-sense amplifier 300 also provides a voltage swing on thebit line/plate line pair that does not interfere with the data stored ina memory cell when data is read from an adjacent memory cell. In thisscheme, a high logic value corresponds to a power supply voltage of 2volts. Additionally, a low logic value on a bit line/plate linecorresponds to approximately 0.7 volts. Thus, the equilibration voltageis selected to be approximately 1.35 volts which is centered between thehigh and low logic values.

In this embodiment, it is possible for node 350-A or 350-B to be reduceddown to -0.6 volts. Thus, in order to prevent corruption of the datastored in a memory cell, word lines WL are biased at -1 volts when off.

Further, transistors 318, 340 and 342 are used in order to establishappropriate voltages on the bit line/plate line pairs as well as toassure a proper voltage swing in sense amplifier 300. The manner inwhich this is accomplished is described in conjunction with FIGS. 6Athrough 6E. Initially, at time t₁, the voltage on isolation transistor310-B and 312-B is reduced to ground, thus isolating sense amplifier 300from memory bank B. At time t₂, the word line of the selected cell formemory bank A is brought to a high voltage level in order to activatetransistor 352. Capacitor 354 thus shares charge with bit line BL, forexample, when a high logic value is stored in the selected memory cell.Also, the voltage on plate line PL is reduced by a corresponding amount.

At time t₃, the signal ISO₋₋ A is reduced to a low logic level thusisolating the sense amplifier from bit line BL-A and plate line PL-A. Attime t₄, NLAT1 and NLAT2 control signals provided to transistors 318 and340, respectively, are brought to a high voltage value in order toactivate the n-sense amplifier 304. N-sense amplifier 304 thus drivesthe low voltage node, in this case node 305, toward ground. By includingtransistor 318 and control signal NLAT1, this initial voltage swing onthe sense amplifier is between V_(cc) and ground which providessufficient drive for n-sense amplifier 304. At time t₅, the controlsignal ISO₋₋ A returns to a high logic value thus reconnecting bit lineBL-A and plate line PL-A to nodes 307 and 305, respectively. With thebit line and plate line reinstated, the NLAT1 signal returns to a groundpotential at time t₆. Thus, from time t₆ forward, n-sense amplifier 304is controlled by transistors 340 and 342 which impose an approximately0.7 volt level on node 305 and thus on plate line PL-A. Thus, in thismanner, sense amplifier 300 provides sufficient voltage swing in senseamplifier 300 in order to adequately latch the data. It also providesappropriate voltage levels on the bit line and plate lines in order toallow dynamic sensing without corrupting data.

Conclusion

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. For example, the high and low logic values selected for thebit line/plate line pairs may be varied from the specified 1 and 2 voltlevels. For example, in a 3 volt part, high and low logic levels may beselected at 3 volts and 1.5 volts with an equilibration voltage of 2.25volts. Similarly, other voltages may be selected as appropriate.Further, equilibration circuits shown in the various embodiments areshown by way of example. Other equilibration circuits that effectivelyshort the bit line and plate line together may be substituted. Further,the teachings of the present invention are not limited to use with ashared sense amplifier.

What is claimed is:
 1. A memory device, comprising:an array of word lines and complementary bit line/plate line pairs coupled to provide addressable access to a number of memory cells; a row decoder and a column decoder coupled to provide addressable access to the memory cells; a sense amplifier coupled to the complementary bit line/plate line pairs; an equilibration circuit that equilibrates a complementary bit line/plate line pair during a read operation at a voltage level that is greater than one-half of the high logic level for the complementary bit line/plate line pairs; and wherein the voltage swing on the sense amplifier is greater than the voltage swing on the bit line/plate line pair between low and high logic levels.
 2. The memory device of claim 1, and further comprising p-channel isolation transistors that couple the bit line and plate line to the sense amplifier so as to limit the voltage swing on the bit line/plate line pairs.
 3. The memory device of claim 2, and further comprising a write-assist circuit coupled to decrease the time for writing a low logic level to one of the bit and plate lines.
 4. The memory device of claim 3, wherein the write assist circuit comprises:first and second transistors; a first source/drain region of the first and second transistors are coupled together at a common node; a second source/drain region of the first transistor is coupled to the plate line; a second source/drain region of the second transistor is coupled to the bit line; a gate of the first transistor is coupled to a node of the sense amplifier; a gate of the second transistor is coupled to a different, complementary node of the sense amplifier; and a voltage reference circuit is coupled to the common node of the first and second transistors such that one of the first and second transistors provides the reference voltage to one of the bit and plate lines to assist in writing a low logic level.
 5. The memory device of claim 1, wherein the sense amplifier includes circuitry that establishes the high logic level for the sense amplifier at a voltage above the power supply voltage.
 6. The memory device of claim 5, wherein the sense amplifier further includes circuitry that establishes a low logic level for the sense amplifier that is above ground potential.
 7. The memory device of claim 5, wherein the isolation devices are n-channel isolation devices.
 8. A memory device, comprising:an array of word lines and complementary bit line/plate line pairs coupled to provide addressable access to a number of memory cells; a row decoder and a column decoder coupled to provide addressable access to the memory cells; a sense amplifier coupled to the complementary bit line/plate line pairs; an equilibration circuit that equilibrates a complementary bit line/plate line pair during a read operation; wherein the sense amplifier comprises: an n-sense amplifier with a first node coupled to the bit line and a second node coupled to the plate line; a p-sense amplifier coupled between the first and second nodes of the n-sense amplifier; a reference voltage source coupled to the n-sense amplifier such that the n-sense amplifier drives one of the bit and plate lines to a reference voltage which reference voltage corresponds to the low logic level for the memory cells; and wherein the p-sense amplifier is coupled to a reference voltage that is above the voltage of a supply voltage for the memory device so as to provide a voltage swing on the sense amplifier that is greater than a voltage swing on the complementary bit line/plate line pair.
 9. An apparatus, comprising:an electronic system; a memory device coupled to the electronic system, wherein the memory device uses dynamic cell plate sensing with ac equilibration of bit line/plate line pairs; wherein the memory device comprises; an n-sense amplifier with a first node coupled to a bit line and a second node coupled to a plate line; a p-sense amplifier coupled between the first and second nodes of the n-sense amplifier; and a reference voltage source coupled to the n-sense amplifier such that the n-sense amplifier drives one of the bit and plate lines to the reference voltage which reference voltage corresponds to the low logic level for the memory cells and wherein the reference voltage source further comprises a second reference voltage which is used initially to drive the n-sense amplifier and the p-sense amplifier to voltage levels such that a voltage swing between the n-sense amplifier and the p-sense amplifier is greater than a voltage swing of the bit line/plate line pairs.
 10. A method for storing data in a selected memory cell of a memory device that uses dynamic cell plate sensing with ac equilibration, the method comprising the steps of:latching data in a sense amplifier of the memory device; converting logic levels of the data in the sense amplifier to different logic levels for the memory cells; storing the data in the selected cell; and wherein latching the data with a sense amplifier comprises driving an n-sense amplifier with a voltage level approximately equal to the low logic level for the memory cell and driving a p-sense amplifier with a voltage level that is above the high logic level for the memory cells such that a voltage swing between the logic levels of the data in the sense amplifier is greater than a voltage swing between the logic levels for the memory cells.
 11. A memory device, comprising:an addressable array of memory cells addressably coupled to word lines and complementary bit line/plate line pairs; a sense amplifier coupled to the complementary bit line/plate line pairs; an equilibration circuit coupled between each complementary bit line/plate line pair that equilibrates the complementary bit line/plate line pair as part of a read operation to a voltage level that is greater than one half of the high logic level for the complementary bit line/plate line pair; and wherein the voltage swing on the sense amplifier is greater than the voltage swing on the complementary bit line/plate line pair between low and high logic levels.
 12. The memory device of claim 11, wherein the high logic level for the complementary bit line/plate line pair is approximately two times the low logic level so as to prevent corruption of data of cells on the same plate line as an accessed cell due to fluctuations on the plate line voltage.
 13. The memory device of claim 11, wherein the sense amplifier is coupled to the bit line/plate line pairs through p-channel transistors so as to establish the lower voltage swing for the bit line/plate line pair.
 14. The memory device of claim 11, wherein the sense amplifier includes a p-sense amplifier that is driven by a voltage that is greater than the high logic level for the bit line/plate line pair.
 15. A memory device, comprising:an array of word lines and complementary bit line/plate line pairs coupled to provide addressable access to a number of memory cells; a row decoder and a column decoder coupled to provide addressable access to the memory cells; a sense amplifier coupled to the complementary bit line/plate line pairs; and an equilibration circuit that ac equilibrates a complementary bit line/plate line pair at an equilibration voltage that is greater than one-half of the high logic level of the memory cells prior to reading data.
 16. The memory device of claim 15, and further comprising p-channel isolation transistors that couple the bit line and plate line to the sense amplifier so as to limit the voltage swing on the bit line/plate line pairs.
 17. The memory device of claim 15, and further comprising a write-assist circuit coupled between a bit line and a plate line and coupled to the sense amplifier so as to decrease the time for writing a low logic level back to one of the bit and plate lines.
 18. The memory device of claim 15, wherein the sense amplifier comprises:an n-sense amplifier with a first node coupled to the bit line and a second node coupled to the plate line; a p-sense amplifier coupled between the first and second nodes of the n-sense amplifier; and a reference voltage source coupled to the n-sense amplifier such that the n-sense amplifier drives one of the bit and plate lines to a reference voltage which reference voltage corresponds to the low logic level for the memory cells.
 19. The memory device of claim 15, wherein the p-sense amplifier is coupled to a reference voltage that is above the voltage of a supply voltage for the memory device so as to provide sufficient voltage range to drive the sense amplifier.
 20. An apparatus, comprising:an electronic system; and a memory device coupled to the electronic system, wherein the memory device uses dynamic cell plate sensing with ac equilibration of bit line/plate line pairs and wherein the equilibration voltage level is selected to be greater than one-half of the high logic level for the bit line/plate line pairs.
 21. The apparatus of claim 20, wherein the memory device includes p-channel isolation transistors that couple plate lines and bit lines to a sense amplifier so as to allow a larger range of voltages on the sense amplifier than on the bit and plate lines.
 22. The apparatus of claim 20, wherein the memory device comprises memory cells with high logic levels that are twice the voltage of the low logic level.
 23. A method for storing data in a selected memory cell of a memory device that uses dynamic cell plate sensing with ac equilibration, the method comprising:latching data in a sense amplifier of the memory device; converting logic levels of the data in the sense amplifier to different logic levels for the memory cells; and storing the data in the selected cell wherein the equilibration voltage is set at a voltage level that is greater than one-half of the high logic level for the memory cells.
 24. The method claim 23, wherein the step of converting logic levels comprises passing the data through a p-channel isolation transistor.
 25. The method of claim 24, wherein passing the data through a p-channel isolation transistor further comprises the step of assisting the p-channel isolation transistor with a write assist circuit that aides in the transfer of a low logic level to a plate or bit line. 